DMOS device structure, and related manufacturing process

ABSTRACT

A DMOS device structure includes a lightly doped semiconductor layer of a first conductivity type, a plurality of lightly doped semiconductor regions of a second conductivity type extending from a top surface of the lightly doped semiconductor layer thereinto, source regions of the first conductivity type contained in the lightly doped semiconductor regions and defining channel regions. The lightly doped semiconductor regions are contained in respective enhancement regions of the lightly doped semiconductor layer of the same conductivity type as, but with a lower resistivity than, the lightly doped semiconductor layer.

This application is a division of application Ser. No. 08/622,695, filedMar. 26, 1996 now U.S. Pat. No. 5,888,042, entitled DMOS DEVICESTRUCTURE, AND RELATED MANUFACTURING PROCESS now pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improved structure forDouble-diffused MOS-technology (DMOS) devices (such as DMOSFETs,Vertical DMOSFETs or “VDMOSFETs”, IGBTs etc.), and to a relatedmanufacturing process.

2. Discussion of the Related Art

A power VDMOSFET is a device including, integrated in the samesemiconductor chip, several hundred or even thousands elementary cellsrepresenting elementary VDMOSFETs connected in parallel in order tocontribute a given fraction to the overall current of the power device.

In its simplest form, each elementary cell includes a first region of agiven conductivity type (P type for an N-channel device, N type for aP-channel one) formed inside a lightly doped semiconductor layer of theopposite conductivity type (N type or P type, respectively); the lightlydoped layer is formed over a heavily doped semiconductor substrate ofthe same conductivity type, in the case of VDMOSFETs, or of the oppositeconductivity type in the case of IGBTs. The first region includes aheavily doped deep body region surrounded by a more lightly doped bodyregion. An annular source region is formed inside the body and deep bodyregions.

A manufacturing process for an N-channel VDMOSFET is described in “PowerMOSFETs: Power for the 80s”, D. Grant and A. Tregida, Solid StateTechnology, November 1985, which is incorporated herein by reference.The process provides for epitaxially growing a lightly doped N typesilicon layer over a heavily doped silicon substrate; performing a fieldoxidation; forming the heavily doped deep body regions; defining activeareas of the device; growing a gate oxide layer over said active areas;depositing and doping a polysilicon layer over the gate oxide layer;defining gate regions by selectively etching the polysilicon layer;forming the body regions and the source regions to define the channel ofthe VDMOSFET; depositing an oxide layer over the entire surface of thechip; defining contact areas in said oxide layer; forming metal layerson the top and bottom surfaces of the chip; and passivating the topsurface of the chip.

More evolved VDMOSFET structures are described in the U.S. Pat. No.5,382,538 and U.S. Pat. No. 4,774,198, both incorporated herein byreference.

For example, in the U.S. Pat. No. 5,382,538 a structure is describedwherein the heavily doped deep body regions are formed inside the morelightly doped body regions, and are self-aligned with the polysilicongate (and thus with the channel regions). A manufacturing processsuitable for obtaining this structure differs from the previouslydescribed process in that both the lightly doped body regions and theheavily doped deep body regions are formed in a self-aligned manner withthe polysilicon gates, and the lightly doped body regions are formedfirst.

A major problem of power VDMOSFETs is that the lightly doped epitaxiallayer, having a significant resistivity, causes the power device to havea high on-state resistance RDSon (the resistance value between drain andsource terminals when the device is in the conductive state). High RDSonvalues result in significant power dissipation.

Furthermore, it is known that power VDMOSFETs which must withstand highdrain-source voltages require highly resistive and thick epitaxiallayers, and that the RDSon value increases rapidly with the breakdownvoltage BV.

In the U.S. Pat. No. 4,974,059 a high power MOSFET structure isdisclosed which is substantially similar to the structure described inthe already mentioned U.S. Pat. No. 5,382,538, but in which the regionsbetween the elementary cells have the same conductivity type of theepitaxial layer but lower resistivity reducing the RDSon value of thepower MOSFET. All these regions are continuous and shallower than thebody regions of the VDMOSFET elementary cells.

In view of the state of the art described, it is an object of thepresent invention to provide a DMOS device structure which allows areduction of the on-state resistance without affecting the breakdownvoltage value.

SUMMARY OF THE INVENTION

According to the present invention, these and other objects are attainedby means of a device structure comprising a lightly doped semiconductorlayer of a first conductivity type, a plurality of lightly dopedsemiconductor regions of a second conductivity type extending from a topsurface of the lightly doped semiconductor layer thereinto, sourceregions of the first conductivity type contained in the lightly dopedsemiconductor regions and defining channel regions, wherein said lightlydoped semiconductor regions are contained in respective enhancementregions of the lightly doped semiconductor layer of the sameconductivity type as, but with a lower resistivity than, the lightlydoped semiconductor layer.

As a result of the present invention, it is possible to reduce theon-state resistance RDSon of a DMOS-technology power device: in fact,the presence of the enhancement regions around the lightly doped bodyregions reduces the major components of RDSon, such as the JFETcomponent Rjfet. Such a reduction in the RDSon is not obtained at theexpense of a reduction in the breakdown voltage: on the contrary,experimental tests have shown that the presence of the enhancementregions increases the breakdown voltage of the device.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will be made moreevident by the following detailed description of some embodiments,described as non-limiting examples in the annexed drawings, wherein:

FIG. 1 is a cross-sectional view of a DMOS device structure according toa first embodiment of the present invention;

FIG. 2 is a diagram showing the doping concentration profiles of somedoped semiconductor regions of the structure of FIG. 1;

FIG. 3 is a diagram of the electric field distribution in the structureof FIG. 1 and in a conventional structure, in breakdown conditions;

FIG. 4 is a cross-sectional view of a DMOS device structure according toa second embodiment of the present invention;

FIG. 5 is a cross-sectional view of a DMOS device structure according toa third embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 is a cross-sectional view of a DMOS device structure according tothe present invention, in particular Vertical Double-diffused MOSFET(VDMOSFET).

Conventionally, a lightly doped semiconductor layer 1 (drain layer) isformed over a heavily doped semiconductor substrate 2, and elementarycells 3 are formed in the lightly doped drain layer 1. The elementarycells have the structure described in the already cited U.S. Pat. No.5,382,538, with a lightly doped body region 4 enclosing a heavily dopedregion 5 and an annular source region 6; the source regions 6 define,inside the respective body regions 4, a channel region. In the case ofan N-channel VDMOSFET, the substrate 2, the lightly doped drain layer 1and the source region 6 are of the N conductivity type, while the bodyregions 4 and the regions 5 are of the P conductivity type. In the caseof a P-channel device all the conductivity types are reversed. Also, thestructure can represent an Insulated Gate Bipolar Transistor (IGBT),either N- or P-channel, provided that the conductivity type of thesubstrate 2 is opposite to that of the lightly doped drain layer 1.

A polysilicon gate 7, insulated from the underlying semiconductorregions by a thin gate oxide layer 8, extends between adjacentelementary cells 3, and is covered by a dielectric layer 9 in whichcontact windows are provided over the central region of each elementarycell 3, to allow a source metal layer 10 to contact the heavily dopedregions 5 and the source regions 6. Also, a drain metal layer 11 isprovided at the bottom of the substrate 2.

The body regions 4 are enclosed within respective enhancement regions 12of the same conductivity type as the lightly doped drain layer 1, butmore heavily doped. FIG. 2 is a diagram (not to scale), showing thedoping concentration profiles (expressed in log. atoms (at) per cubiccentimeter) of the various doped semiconductor regions of the structureof FIG. 1, as a function of the distance x from the semiconductorsurface. It can be appreciated that the concentration of N type dopantsnear the junction between the P type body region 4 and the drain layer 1is higher than in conventional structures, where the enhancement regions12 are absent.

The presence of the enhancement regions 12 around the body regions 4 hasseveral advantages, which will be now discussed.

Firstly, the on-state resistance RDSon of the VDMOSFET is reduced. Infact, the presence of the enhancement regions 12 increases the overallamount of dopant ions in the portions of the drain layer 1 comprisedbetween adjacent elementary cells (thus determining a reduction in theso-called Rjfet component of the RDSon), and creates a preferred pathfor the flow of electrons towards the substrate 2, thus determining areduction of the so-called Rdrift component, associated to the drainlayer 1, of the RDSon; the Rdrift component of RDSon, for VDMOSFETs ofmedium/high voltage (with breakdown voltage BVDSS>250 Volts), is themost important component.

The reduction in the Rjfet component of RDSon allows to reduce thespacing between adjacent cells, which translates in an increase of thecell density. This increases the channel length per unit area, andreduces the value of the gate-drain parasitic capacitance.

As the body junction depth is reduced, so is the channel length, andcorrespondingly the R channel component of the RDSon.

Secondly, the presence of the enhancement regions 12 increases thebreakdown voltage BVDSS of the VDMOSFET. This can be appreciatedreferring to FIG. 3. In this figure, the electric field profile alongthe drain region 1 at the breakdown as a function of the distance x′from the junction between the body region 4 and the drain layer 1 isshown in two different cases. Curve A refers to a conventionaluniformly-doped semiconductor layer 1, with a dopant concentration of2*10¹⁴ atoms/cm³ (resistivity equal to 22 Ohms*cm), typical of aVDMOSFET with a breakdown voltage BVDSS of 500 Volts. Curve B refers tothe structure of the present invention, with a semiconductor layer 1having the same dopant concentration as in the case of curve A, butwherein enhancement regions 12 are provided.

In the case of curve A, the electric field E reaches its maximum valueEcrit (at which breakdown occurs) at the junction between the bodyregion 4 and the drain layer 1 (x′=x′a), and then decreases linearly,with a sloped-dE/dx′, moving towards the interface of the drain layer 1with the substrate 2, where the field has the value Ecrit-W*dE/dx′ (Wbeing the thickness of the so-called “residual drain layer”, i.e. thedistance between the substrate 2 and the junction between the bodyregion 4 and the drain layer 1).

In the case of the present invention, the electric field E does notdecrease linearly with the distance x′ from the edge of the body region4, and is always higher than in the case of curve A. Breakdown takesplace either when the electric field reaches the value Ecrit at thepoint P, shown in FIG. 2, where the dopant concentration of theenhancement region 12 becomes negligible compared with the dopantconcentration of the drain layer 1 or when the electric field value atthe body/drain junction (x′=0) exceeds the value Ecrit of theenhancement region 12, whichever occurs first. Point P is located atsome microns (x′p) from the junction between the body region 4 and theenhancement region 12.

The increment ΔBV in the breakdown voltage value thus obtainedcorresponds to the increase in the area subtended by the curve of theelectric field. Approximating the portion of curve B in the regioncomprised between x′=0 and x′=x′p with a straight line, we obtain:

ΔBV=(E max−Ecrit)*x′p/2+x′p*(W−x′p)*dE/dx.

It is evident that, instead of having a VDMOSFET structure which, with adrain layer 1 of a given thickness, has an higher BV, it would bepossible to have a VDMOSFET which, for a given value of BV, has athinner drain layer 1, and thus a lower RDSon.

FIG. 4 shows in cross-sectional view a second embodiment of the presentinvention. In this embodiment, the VDMOSFET has a “stripe” structure,instead of a “cellular” one. This means that the body regions areelongated stripes 13, instead of square or hexagonal cells as in FIG. 1.Also, the heavily doped regions 5 are replaced by heavily doped stripes15, and the source regions are represented by stripes 16; it is to benoted that with a stripe geometry, it is not necessary to provide acentral area wherein the source region is absent for the contact to theheavily doped region 15: it is sufficient to provide periodicalinterruptions in the source stripes 16, or alternatively to merge allthe heavily doped regions 15 along the periphery of the chip. In thisway the integration density can be increased. As visible from FIG. 4,and according to the present invention, the body stripes 13 are formedwithin respective enhancement stripes 14 of the same conductivity typeas the drain layer 1, but more heavily doped.

FIG. 5 shows a third embodiment of the present invention. This thirdembodiment relates again to a VDMOSFET with cellular structure, but inwhich heavily doped regions 17 deeper than body regions 18 are provided;for this reason, the heavily doped regions 17 are also called “deep bodyregions”. The depth of enhancement regions 19 is intermediate betweenthe depth of the deep body regions 17 and that of the body regions 18.

When compared with the structure of U.S. Pat. No. 4,974,059, there is asignificant advantage in that the enhancement regions associated witheach cell are clearly separated from each other, they do not extendbelow the entire extension of the polysilicon gate electrode 7 and thusthe parasitic capacitance between gate and drain of the DMOS device isreduced.

A process for the manufacturing of a DMOS device structure according tothe present invention is totally similar to the conventional processes(such as that described in the already cited U.S. Pat. No. 5,382,538, orin the above mentioned technical paper by Grant and Tregidga), exceptfrom an additional step involving an implantation of a dopant for theformation of the enhancement regions 12.

More specifically, the structures shown in FIGS. 1 and 4 can bemanufactured by means of the process described in the U.S. Pat. No.5,382,538, while the structure of FIG. 5 can be manufactured by means ofthe process described in the technical paper by Grant and Tregidga.

In both cases, the implantation step is performed after the definitionof the polysilicon gates, and before the implantation of a dopant forthe formation of the body regions 4, 13 or 18. Considering for examplethe case of an N-channel device, a suitable dopant for forming theenhancement regions 12, 14 or 19 is phosphorus, and the implantationdose can be of 5*10¹² ions/cm2. A thermal diffusion process is performedafter the implantation of phosphonousions. Alternatively, it is possibleto perform a unique thermal diffusion process after the implantation ofthe P type dopant (boron) for the formation of the body regions, takingadvantage of the higher diffusivity of phosphorus with respect to boronto obtain body regions enclosed within the enhancement regions. Theenhancement regions are self-aligned with the polysilicon gates.

The enhancement regions 12, 14 or 19 can be defined selectively, bymeans of known photolithographic techniques, or non-selectively, withthe polysilicon gates and the field oxide (not shown in the drawings)preventing the formation of enhancement regions in regions differentfrom the elementary cells 3 or stripes 13.

The DMOS structure of the present invention can be used not only indiscrete devices, but also in Power Integrated Circuits (PICs).

Having thus described at least one illustrative embodiment of theinvention, various alterations, modifications, and improvements willreadily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be within the spirit andscope of the invention. Accordingly, the foregoing description is by wayof example only and is not intended as limiting. The invention islimited only as defined in the following claims and the equivalentsthereto.

What is claimed is:
 1. Process for the manufacturing of a DMOS device,comprising the steps of forming a lightly doped semiconductor layer of afirst conductivity type over a heavily doped semiconductor substrate,forming a gate oxide layer over the lightly doped semiconductor layer,forming a conductive gate layer over the gate oxide layer, selectivelyremoving the conductive gate layer to define conductive insulated gates,forming lightly doped semiconductor regions of a second conductivitytype in a self-aligned manner with said conductive insulated gates,wherein before forming said lightly doped semiconductor regions, itprovides for forming enhancement regions of the same conductivity typeas but with a lower resistivity than said lightly doped semiconductorlayer, said enhancement regions being formed in a self-aligned mannerwith said conductive insulated gates.
 2. Process according to claim 1,wherein after the definition of said conductive insulated gates, itprovides for implanting a first dopant of the first conductivity type,thermally diffusing said first dopant to form the enhancement regions,implanting a second dopant of the second conductivity type, andthermally diffusing the second dopant to form the lightly dopedsemiconductor regions inside respective enhancement regions.
 3. Processaccording to claim 1, wherein after the definition of said conductiveinsulated gates, it provides for implanting a first dopant of the firstconductivity type, implanting a second dopant of the second conductivitytype, the first dopant having a higher diffusivity than the seconddopant, and thermally diffusing the first and second dopant to form thelightly doped semiconductor regions contained in the enhancementregions.
 4. Process according to claim 2, wherein the implant of saidfirst dopant of said first conductivity type is performed selectively.5. Process according to claim 2, wherein the implant of said firstdopant of said first conductivity type is performed without performingadditional photolithographic steps for defining additional patterns. 6.Process according to claim 3, wherein the implant of said first dopantof said first conductivity type is performed selectively.
 7. Processaccording to claim 3, wherein the implant of said first dopant of saidfirst conductivity type is performed without performing additionalphotolithographic steps for defining additional patterns.